Electrical power is distributed from central generating plants to homes, offices, and factories as three phase alternating current. It has long been realized that the choice of AC, rather than DC, allows the use of transformers that permit power to be distributed at higher voltages than the voltage at which it is ultimately used. This practice reduces the current in the distribution system and thereby maximizes distribution efficiency by minimizing losses. To improve efficiency, and to make high starting torque AC motors simpler to engineer, the power transmission system is three phase.
In recent years there has been an increasing public concern about possible biological effects of the low frequency electrical and magnetic fields associated with the distribution and use of electrical power. One can find, for example, a conjectural review of possible phenomenological bases for such effects as well as a discussion of a number of recent epidemological studies in an article by Karen Fitzgerald, et. al., in the August, 1990 issue of IEEE Spectrum (ISSN 0018-9235). Although the evidence in this area is far from being unequivocally persuasive, the level of public concern has led to litigation aimed at preventing the construction of transmission powerlines. For example, the State of Florida, in Chapter 17-274 of the Florida Administrative Code, has a guideline limiting both electric field strength and magnetic flux density at the edge of right of way for new powerlines. These two quantities are measured in accordance with ANSI/IEEE 644-1987 Standard Procedures for Measurement of Power Frequency Electric and Magnetic Fields from AC Power Lines.
In discussing public health issues related to power lines, it is convenient to consider three classes of powerlines:
1) Urban distribution lines running from a substation to distribution transformers located near the point of use, and commonly operated at 10 to 50 kV.
2) Urban transmission lines that supply power to the substations, that are usually not isolated on rights of way, and that are commonly operated at 69 to 230 kV .
3) Rural transmission lines that are on rights of way and that are commonly operated at voltages above 230 kV.
Although much of the most recent public outcry has been directed at the third category of power distribution lines listed above, the 10-50 kV category may be more significant because of the vastly greater number of people exposed. Distribution lines in the 10-50 kV category, if mounted on poles, can give rise to measured electrical fields of as much as 20 V/m, and magnetic fields as high as 1.3 micro-tesla when measured on the ground below the lines.
The issue of fringing fields form 10-50 kV powerlines has a well known solution--underground utilities. Both theory and measurements show that placing all three phase conductors in an underground common conduit provides nearly perfect magnetic and electrostatic shielding. There are also aesthetic advantages to underground utilities, which has led to their widespread use in new construction. A major question in the controversy over possible health risks from fringing fields is whether to rebuild existing distribution lines and to literally bury the supposed problem. A method of reducing or eliminating fringing fields while using existing pole-supported open wires could provide an economically attractive alternative method of resolving the problem.
In many larger cities of the United States, the second level of electrical distribution is served by 69 kV to 138 kV lines on wooden or steel poles that are located along streets rather than being on dedicated rights of way. It is common to measure electrical fields of over 100 V/M in the front yards of homes that are adjacent to such lines. The magnetic fields associated with these lines, however, are usually lower than those from the lower voltage 10-50 kV distribution lines that commonly mounted below the 69-138 kV circuit on the same poles.
The intermediate distribution system of category 2 may pose a serious potential problem. Theoretically, burying the three conductors in a common steel or plastic conduit, which is commonly used for the 10-25 kV lines, will work at any voltage level, and will eliminate fringing electrical fields. If the same wiring geometry is used for these lines as is employed for the 10-25 kV lines, burial will also practically eliminate fringing magnetic fields above ground. Putting all three conductors in a common steel conduit unfortunately poses other problems, partly because of the difficulty of providing adequate phase-to-phase dielectric insulation at the higher voltages. In addition to the insulation question, the added line capacitance associated with this construction could lead to transmission inefficiencies because of leading power factor. If, on the other hand, the three phase conductors are buried in separate conduits and the common neutral wye connection is used for the transformers at either end of the line, fringing magnetic fields are not eliminated. In fact, since people walking above the buried line are closer than they would be to a comparable overhead power line, exposure to magnetic fringing fields could be more severe for an underground line.
At larger substations, power from generating plants is supplied at 230 kV or higher. These lines are usually constructed on dedicated rights of way that are typically 60 m (about 200 ft) wide. For remote power plants and interconnection with other utilities, the powerlines are operated at 345 kV, 500 kV, and 765 kV. These high capacity powerlines have large steel towers, long insulators, and bundles of conductors. Fortunately, the electrical and magnetic fields from these lines diminish rapidly in intensity the further one goes away from the conductors and the number of people who live within 200 m of these power lines is small; thus, it is not as significant a potential public health risk as the other two categories.
One can show from basic physical theory that the strength of electrical and magnetic fields decreases dramatically with distance from the two parallel conductors that are commonly used for single phase power distribution. (Magnetic intensity falls inversely with the square of distance). Near the conductors, the electric and magnetic fields are intense, but once one moves further away from the line than 5 times the conductor-to-conductor spacing, the fields are weak. Although fringing fields can be reduced by close conductor spacing, this approach is limited by other factors, such as the dielectric strength required to prevent arcing between the phase conductors.
The foregoing discussion is directed toward transmission lines made of several parallel conductors. Another widely used transmission line design is "coaxial", which is widely used when shielding is important. The adaptation of coaxial lines to the field-free transmission of electric power has been discussed in my co-pending application (U.S. Ser. No. 07/578,215)
Coaxial transmission lines have a higher capacitance per unit length than do parallel lines, and will hence have a higher leading power factor. In a 10-50 kV distribution line this may not be a problem, as both fluorescent lights and induction motors for air conditioning operate with a lagging power factor that the coaxial line may offset.
A different assignment of conductor phase angles for six-phase transmission was disclosed by Y. Onogi, F. Isaka, A. Chiba, and Y. Okumoto, in their article "A Method of Suppressing Fault Currents and Improving the Ground Level Electric Field in a Novel Six-Phase Power Transmission System," which appeared in the IEEE Transactions on Power Apparatus and Systems, PAS-102, p 870-880, 1983. In the teaching of Onogi et al the phase angle difference between the three vertically disposed conductors on each side of a support tower is 120 degrees. The phase angle difference between horizontally disposed pairs of conductors is 180 degrees. Onogi, et. al., illustrate the significant reduction in ground level electric field as compared to the phase angles taught by Stewart, et. al. The magnetic field measured 1 m above ground is also lower for the Onogi, et. al., phase angle choice when compared to the Stewart, et. al, phase angle choice.
At higher voltages the higher capacitance of coaxial lines becomes a serious disadvantage. In general, the leading current per phase is given by V.sup.2 /C, where V is the phase voltage and C is the capacitance. Increases in line voltage may be accompanied by a decrease in capacitance, caused by an increased thickness of dielectric, but this contribution to the reactance will be dominated by the second power of the voltage increase--i.e. the leading current per phase will increase proportionally to about the 1.8 power of voltage. At 230 kV and above, both the cost and the excess capacitance of a coaxial transmission line rule it out for long distance powerlines.
Another relevant area of technical background to the present invention is the use of six phase transmission to increase the amount of electrical power that can be transmitted along a given width right of way. The basic idea behind this work was to replace an existing dual circuit three phase powerline with the same phase to ground voltage six phase powerline and increase power transmission by 73%. Initial work in this field, which was reported by James R. Stewart, et. al, in the October 1985 Transmission & Distribution magazine (ISSN 0041-1280), arranged the six phase conductors symmetrically. A cross section of this power line was a hexagon, with one conductor at each vertex and with the phase advancing by 60 degrees as one went around the hexagon from conductor to conductor. The electric field and magnetic flux density on the ground under and near the six phase powerline are comparable to those from the replaced three phase powerline. Although this arrangement did demonstrate that six phase transmission offered a way to increase the amount of electrical power that could be sent down a given right of way, it involved an expensive construction technique which is suited to rebuilding existing powerlines located on narrow dedicated rights of way. This prior art did not show a significant reduction to the fringing electric and magnetic fields.
This defines the problem to be solved by my invention: Significantly reduce both fringing magnetic fields and electric fields of an open wire high voltage multiphase power transmission line by compatible means that are adaptable to rebuilding existing lines in urban areas as well as rural areas.